1. Field of the Invention
This invention relates to a novel bipolar electrolytic cell in which the conductor resistance of electrolytic cell is very small, replacement of the deteriorated anode is easy, and damage to the electrolytic cell is very slight when trouble occurs.
Use of such a bipolar electrolytic cell reduces the requisite electrolytic voltage for electrolysis, facilitates the replacement of a deteriorated anode, and greatly simplifies the repair of the electrolytic cell when trouble occurs.
2. Description of the Related Art
There are various known electrolytic cell structures for industrial electrolysis of brine, etc. In industrial electrolysis, a reduction in electrolytic voltage is naturally important. From this viewpoint, it is desirable for conductor resistance of the electrolytic cell to be small. Further, it is desirable for the electrolytic-cell structure in which the anode can be easily replaceable when it deteriorates as a result of degradation, damage, etc. Also, it is desirable for the electrolytic-cell structure to be minimally liable to damage when trouble occurs.
The related art of the present invention will be described with reference to an electrolytic cell used in electrolysis of an alkali chloride aqueous solution.
In the method commonly referred to as the "ion-exchange membrane salt electrolysis" method, electrolysis is performed on brine by using a fluorine-containing cation-exchange membrane to produce chlorine at the anode, and caustic soda and hydrogen at the cathode. As is generally well known, this method provides a higher energy efficiency than that of the conventional mercury or diaphragm method and also enables a caustic soda of a higher purity to be produced than that of the conventional mercury or diaphragm method.
In recent years, there is an increasing demand for energy saving, with the result that it is becoming more and more necessary to realize a still higher energy efficiency for salt electrolysis by the ion exchange membrane method. In view of this, a technique is being developed which makes it possible to conduct salt electrolysis with a lower electrolytic voltage and/or a higher current efficiency.
Usually, salt electrolysis is conducted with an electrolytic voltage which is as much as 1V or more higher than the theoretical electrolytic voltage of approximately 2.2V. As is well known, this is attributable to the overvoltages at the anode and at the cathode, and to the increase in voltage due to the various resistance components, such as membrane resistance, solution resistance and conductor resistance. That is, to achieve a reduction in electrolytic voltage, it is important to realize a reduction in the anode overvoltage, cathode overvoltage, membrane resistance, solution resistance, electrolytic-cell conductor resistance, etc.
At present, an electrode consisting of a titanium base coated with a catalyst having a low-chlorine overvoltage characteristic, such as ruthenium oxide, has been put into practical use and is being widely used as the anode. For the cathode, various electrodes having a low-hydrogen overvoltage characteristic have been devised and put into practical use. Due to these techniques, the anode and cathode overvoltages have been overcome to a considerable degree.
Since the catalytic activity at the anode gradually deteriorates as electrolysis proceeds, it is necessary to replace the electrode catalyst by a new one after long use. Usually, the replacement of the electrode catalyst at the anode is carried out by replacing the anode with a new one.
As a cation exchange membrane, a fluorine-containing cation exchange membrane is in use which consists of a fluorine-resin body combined with carboxylic acid and/or sulfonic acid groups as fixed ions. This membrane has been improved by reducing the electrolytic voltage and enhancing the current efficiency and durability. As a result, it has been considerably improved as far as reduction in membrane resistance and enhancement in current efficiency are concerned.
Thus, in recent years, all attempts to improve electrolytic cells have been directed to reducing the solution and conductor resistances through an improved electrolytic method, an improved electrolytic cell, etc. As a means for attaining a reduction in solution resistance, a method has been proposed according to which electrolysis is conducted with the cathode and the anode being set as close as possible to each other; that is, the membrane and the anode are placed in close contact, and so are the cathode and the membrane. To mitigate the increase in electrolytic voltage due to adhesion to be membrane surface of the bubbles generated during electrolysis, a membrane is generally used which has undergone a bubble-opening treatment so as to prevent adhesion of bubbles to the membrane surface on the anode and/or cathode surface.
Electrolytic cells for industrial use are roughly divided into monopolar and bipolar types. In comparison with the monopolar type, the bipolar type electrolytic cell has a number of advantages. For example, its structure can be made relatively simple since it does not require wiring for each cell. Further, since it does not need a large current, the rectifier, bus bar, etc. can be compact and inexpensive. Thus, it leads to a great industrial advantage to achieve a reduction in conductor resistance in bipolar electrolytic cells.
Conductor resistance greatly depends on the structure, material, etc. of the electrolytic cell. That is, conductor resistance is determined by the path of current through the electrolytic cell and by the specific resistance of the electrolytic cell material used in the path of current. Therefore, it is possible to attain a reduction in conductor resistance by making the current path as short as possible and/or employing a material having a small-specific resistance for the current-path portion of the cell.
However, an electrolytic cell for alkali-chloride electrolysis has a problem in that while its cathode may be made of a material having a relatively small specific resistance, such as nickel or an iron-based alloy, its anode is generally formed of a material such as-titanium or a titanium-based alloy, which, though these materials have good durability against chlorine gas, has a relatively large specific resistance. Further, in a conventional bipolar electrolytic cell, a substantially flat titanium plate is used for the partition on the anode side, and the partition and the anode are electrically and mechanically connected to each other by means of conductive ribs made of titanium. Further, to ensure a passage for uniformalizing the solution in the anode chamber and for discharging the generated chlorine gas to the exterior of the electrolytic cell, a gap of 30 to 50 mm is generally required between the anode and the partition on the anode side.
That is, in the conventional bipolar electrolytic cell, an increase in voltage occurs due to the so called conductor resistance when current flows through the conductive ribs on the anode side, which has a large specific resistance. In view of this, there has been a demand for a bipolar electrolytic cell capable of reducing, in particular the conductive resistance on the anode side.
In order to meet this demand, the present applicant proposed, in Japanese Patent Laid-Open No. 58-71382, an electrolytic cell in which, with a view toward reducing the conductor resistance of the electrolytic cell, the cathode and anode chambers were made of a thin plate of a corrosion-resistant metal having a corrugated surface configuration.
FIGS. 4 and 5, on page 5 of the above laid-open publication supplied sectional views of an electrolytic cell according to a preferred embodiment of that invention. The electrical connection between the cathode and anode chambers of that electrolytic cell was realized by pressure welding, thereby eliminating the voltage loss which would have been involved if a connector had been used. Further, since the anode was directly connected to protrusions of the partition on the anode side, it was also possible to eliminate the voltage loss which would have been involved if conductive ribs had been used. Thus, the above invention, made by the present applicant and disclosed in the above laid-open publication, has made it possible to attain a substantial reduction in what is called conductive resistance.
However, the present inventors found the following serious problem with the above electrolytic cell when they used it in electrolysis of an alkali chloride.
The partition on the anode side is liable to be damaged when the anode is detached and replaced with a new one after the electrode catalyst has deteriorated after long use. Further, the attachment of the new anode requires much effort. In addition, after a long period of alkali-chloride electrolysis, pin holes are generated in the ion exchange membrane, which may lead to considerable corrosion of the cell portions around the pin holes, e.g., the anode, and the partition on the anode side. Such damage on the anode side of the electrolytic cell can only be made good by entirely replacing the anode-side partition of the electrolytic cell with a new one, which requires much effort and material.